Production of combustible gas



Aug. 17, 1965 J. 5. NEGRA ETAL 3,201,215 I PRODUCTION OF COMBUSTIBLE GASFiled June 7, 1963 JOHN s. NEGRA ARNOLD R. BERNAS INVENTORS.

mpg-$44!? AGENT United States Patent 3,201,215 PRODUCTHQN 0FC(EMEUSTEEILE GAS John S. Negrti, South Ptainfield, and Arnold R.Berries, Nixon, N..I., assignor to Chemical Construction Corporation,New York, N.Y., a corporation of Delaware Filed June '7, 1963, Ser. No.286,296 7 Claims. (Cl. 43-215) This invention relates to thegasification of naphtha with steam andair, to produce astablecombustible gas steam which may be further processed to a finished towngas with low content of illuminates such as ethylene. Naphtha isgasified according to the process of the present invention without theconcomitant formation of free carbon, by the suitable selection ofseveral inter-related processvariables. The reaction of naphtha withsteam andair is controlled so asto produce a crude gas steam containinga substantial proportion of lower hydrocarbons as Well as carbonmonoxide and hydrogen. Unsaturated low hydrocarbons are hydrogenated ina preferred embodiment, so as to produce a stable town gas.

Naphtha is a relatively volatile petroleum refining product orintermediate, which is generally defined in terms of boiling range.Thus, according to a crude oil survey in Industrial and EngineeringChemistry, 44, #11 (Nov. 1952), p. 2578. naphtha is defined as follows:Naphtha content (of crude oil) is the total distillate recovered in theUS. Bureau of Mines routine analysis at a vapor ten perature of 392 F. Amore detailed definition of naphtha appears in Petroleum Refining withChemicals by Kalichevsky and Kobe (1956). A discussion of naphtha on p.2123 of this text indicates that different naphthas may have boilingranges from a low point of 122 F., to a maximum of 400 F. Thus naphtha"is defined as a general term which is applied to fractions boiling inthe gasoline of low kerosene range. In general then, naphtha is alow-boiling and readily volatilized liquid hydrocarbon cut, derived fromcrude oil distillation in petroleum refining. This material consistsmostly of straight chain parafiinics in C to C-9 range, however, uptoabout 30% naphthenics together with up to 10% aromatics andunsaturates may also be present. In addition, naphtha also generallycontains a significant proportion of sulfur in the form of COS andmercaptans.

Naphtha may be utilized in a variety of Ways. Thus, crud-e naphtha maybe further refined and upgraded to yield a variety of finished petroleumsolvents. In many refineries, naphtha is reformed in the petroleum senseof the term. In this case, the crude naphtha is cracked,

and hydrocarbon molecules are re-assembled in the presulfur naphtha isdescribed in US. Patent No. 2,711,419. ireheated naphtha vapor, air andsteam are reacted in the presence of a nickel-type catalyst. Accumulatedcarbon and sulfur deposited on the catalyst are removed by a pcriodicburning-oft period. lumerous other prior art procedures for thecatalytic or non-catalytic gasification 0f naphtha have been proposed inthe prior art. Generally speaking, the formation of free carbon, tarsand gums remains as a problem, principally because of the difiiculty ofcracking the naphtha to form lower hydrocarbons without the concomitantformation of these undesirable side-products.

in the present invention, naphtha is gasified With steam and air atelevated temperature, to produce a crude combustible gas. After furthertreatment such as hydrogenation of unsaturates, a finished town gas isproduced. A limited amount of process air is employed in thegasification stage of the present invention, to assist in thegasification of the naphtha and to provide a nitrogen gas component forballast in the finished town gas. The process of the present inventiondepends on a unique balance of reaction conditions to achieve thegasification of naphtha, since this is accomplished Without accumulateddeposition of free carbon. In addition, no significant amount ofunreacted hydrocarbon is present in the finalsynthesis gas. The basicprocess is carried out in a gasificaion-conditioning stage followed by aquench step. It has been found that the gasification stage must be ofshort duration in order to prevent the reactants from reachingequilibrium with resultant deposition of free carbon in the catalystbed. It has also been found that the reac tants must be preheated inorder to provide a minimum temperature level in the gasification stage.Thus, in the present invention, process streams of naphtha, steam andair are preheated and mixed. A partial reaction ensues, and the mixedprocess stream, now containing a varmonoxide and carbon dioxide.

sence of platinum or other suitable catalyst, so as to yield Y asubstantial proportion of branched chain or aromatics molecules. Thismaterial is then blended with other refinery cuts for gasoline usage.

Naphtha has also been utilized in the prior art as a hydrocarbon rawmaterial for the manufacture of a combustible gas, such as town gas. Animproved process for the gasification of naphtha to produce acombustible town gas in provided in the present invention. Town gas isgenerally defined as a stable gas stream free of gums or tars andcontaining saturated lower hydrocarbons,

;hydrogen, carbon monoxide and inert ballast gas component, with aheating valve in the range of 400 to 1000 "B.t.u./ft. and a specificgravity (air=1) of 05:0.05. A typical prior art procedure for producinga-combustible gas such as town gas from naphtha is described in BritishPatent No. 923,385. In this process, naphtha or light distillate" isreformed with steam, in the presence of a specific catalyst composition.It is claimed that the formation of free carbon, tars and gums isprevented. A cyclic process for the production of fuel gas from highietyof intermediate components but not in final reaction equilibrium, ispassed to a quench step. The resulting crude combustible gas streamconsists of a gas mixture containing methane, ethylene, hydrogen,nitrogen, carbon The stream is essentially free of unreactedhydrocarbons or solid particulate carbon.

The process of the present invention possesses several significantadvantages. A primary advantage is that naphtha is completely convertedto a stable crude combustible gas without the concomitant accumulationof free carbon or tars. No recycle or said stream disposal is required.The process is continuous rather than cyclic or intermittent. Theprocess is non-catalytic in the gasification stage, however as willappear infra a nickel catalyst may be provided if desired to increasethe B.t.u. value of the final town gas.

it is an object of the present invention to produce a combustible gas bythe gasification of naphtha.

Another object is to completely gasify naphtha in a continuous process,without accumulated deposition of tars or free carbon.

A further object is to react naphtha with steam and air Without reachingfinal reaction equilibrium, under conditions such that a stableintermediate stream containing lower hydrocarbons, carbon monoxide andhydrogen is produced.

An additional object is to produce a stable combustible town gas by thenon-catalytic gasification of naphtha.

Still another object is to simultaneously crack, partially oxidize, andnon-catalytically steam reform naphtha by reaction with steam and airunder process conditions such that free carbon is not formed. a i

These and other objects and advantages of the present invention willbecomejevident from the description which follows. Referring to thefigure, stream 1 is a liquid naphtha, derived from petroleum refining orother ty es of crude oil processing. Thus, as described supra, stream 1consists principally of parafinic hydrocarbons in the C5 to C-9 range,together with naphthenics as well as minor amounts of aromatics andsulfur compounds. The liquid stream it is vaporized and preheated inheater 2, to form naphtha vapor stream 3. Vapor stream 3 may be producedat any suitable temperature, ranging from the boiling point of naphthaup to about 1000 F. Above this temperature level the naphtha vapor maybecome unstable, and certain portions or components will readily crackto smaller molecules with concomitant carbon deposition. Stream 3 thusis preferably produced at a temperature ranging from 400 F. to 800 F.

Stream 4 consists of highly superheated steam, preheated usually to atemperature above l500 F. and preferably to the range of 1500 F. to 1700F. Although lower ratios are feasible, it has been found that a range ofmolar steam/ carbon ratios between 3 and 6 is desirable in proportioningthe relative flow rates of streams 4 and 3, in order to assure theavoidance of formation or accumulated deposition of carbon under normaloperating conditions.

Stream 5 consists of air, preheated usually to a temperature above 800F. and preferably to the range of 800 F. to 1200 F. The proportion ofair employed in the process is quite small, thus only enough air is usedto provide the desired thermal effects of temperature elevation due tonaphtha combustion. The streams and 5 are preferably combined, to form amixed steam-air stream 6 at a temperature of at least 1100 F. Stream 6is now combined with naphtha vapor stream 3, and the mixed stream 7 isimmediately passed into residence or gasification chamber 8. It will beappreciated that streams 3, 4 and 5 may be separately passed intochamber however premixing of the air and steam to form stream 6 is apreferable procedure since this results in better and more rapid mixingof the several streams. Thus, stream 3 is more rapidly dispersed anddiluted due to the mixing with stream 6, prior to entry of the naphthavapor into residence chamber 8. Consequently, the possibility oftransient carbon formation or deposition due to cracking of the naphthais reduced by the pre-mixing step.

In chamber 8, simultaneous reactions take place between and among theseveral reactants and intermediate components. The temperature of theprocess stream immediately rises, due to exothermic combustion of aportion of the naphtha with the oxygen content of the air. In addition,a portion of the naphtha is non-catalytically steam reformed due to thehigh temperature and high steam concentration in chamber 3. Thisendothermic reaction serves to produce free hydrogen, and also moderatesthe temperature rise due to combustion. A further portion of the naphthais thermally cracked to lower hydrocarbons, due also to the hightemperature level. However, simultaneous deposition and accumulation offree carbon does not instantaneously occur. Instead, the unstable lowerhydrocarbons are selectively hydrogenated to a certain extent due to thein situ formation of hydrogen, which may possibly be formed in thenascent state. Thus, the resultant gaseous stream 9 contains significantproportions of steam, nitrogen, hydrogen, carbon dioxide, carbonmonoxide unsaturated hydrocarbons (mostly ethylene), methane andpossibly ethane. It should be understood, however, that some of thesecomponents are present on a transient or instantaneous basis. If streamis allowed to reach stable equilibrium under these process conditions,significant formation and accumulated deposition of free carbon willtake place.

Under some conditions, the temperature in unit 8 may be in the range of1450 F. to 1500 F. However, with such low reaction temperatures, theformation of free carbon may readily occur, especially after all theresidual free oxygen is consumed, unless the residence time is kept I)oxide and hydrogen.

in the range of 0.05 to 0.33 second. In general, the residence time inchamber 8 must be kept below 1.0 sec- 0nd, and preferably in the rangeof 0.05 to 0.33 second, in order to achieve the desired reactionswithout carbon formation. In addition, the instantaneous mix temperatureof stream '7 must be kept above 1000 F. since it has been found inpractice that the various competing reactions will tend to form freecarbon if the initial mix temperature is below 1000" F. This initial orinstantaneous mixture temperature should preferably be in the range of1400" F. to 1700 F. in order to preclude carbon formation due to processupsets. It will be evident that chamber 8 may actually, in terms ofapparatus design, consist merely of an insulated pipe section extendingbetween the point of mixing of the reactant streams and the entry of thegasified process stream into the following quench.

In summary, the competing reactions of naphtha combustion, cracking andnon-catalytic thermal steam reform are carried out in the first stage ofthe process of the present invention. It has been determined that, bymaintenance of reaction conditions within certain critical ranges, thesecompeting reactionsmay be carried out without carbon accumulation. Inaddition, the resulting unstable process stream, when passed to thefollowing quench step before further reaction ensues, is successfullystabilized to yield a crude combustible gas mixture without carbonaccumulation. The present invention essentially accomplishes thegasification of naphtha by a process using preheated air and steam. Ithas been determined that the resulting mixed gas stream may besuccessfully maintained as a stable crude combustible gas, if the mixedgas stream is quenched before final process equilibrium is reached.Thus, the critical features of the present invention essentially involvethe maintenance of several inter-related process variables withinoperating limits in which the new result of the present invention isachieved, namely the continuous gasification of naphtha.

Stream 9 now passes into the quench unit 10, in which the gas stream isquenched to a temperature below 800 F, so as to terminate the reactionbefore final equilibrium is attained, and thereby prevent carbonformation. Quench liquid stream 11 is any suitable coolant liquid andmay consist of a stable heavy hydrocarbon oil, however stream 11 willpreferably consist of water. Warmed quench liquid is removed via 12. Thecooled process gas stream is removed via 13 at a temperature below 800F., and consists of a crude combustible gas typically containinghydrogen, carbon monoxide, ethylene, methane, carbon dioxide andnitrogen. Stream i3 is directly usable as a gaseous fuel in certainapplications, however stream 13 is preferably further processed tosaturate any unsaturated hydrocarbons such as, ethylene, which act asilluminants and thus are undesirable in town gas. In addition, it isgenerally desirable to subject the gas stream to the water gas shiftreaction in which carbon monoxide is converted to carbon dioxide byreaction with steam, in order to increase the hydrogen content of thegas stream. Carbon dioxide is also usually removed from the gas stream,in order to increase the Btu. content by decreasing the ballast orinerts content.

Stream 13 is first passed into water gas shift converter 14-, togetherwith further steam which is supplied via 15 when required, to react withthe carbon monoxide; content of stream 13. In some cases stream'i5 maybe omitted, if sufficient steam is already present in stream 13.Converter, 14- is provided with at least one bed to" of catalyst,usually consisting of active iron oxide plus alkali oxide promoter,deposited on a suitable carrier. Unit 14 may consist of the apparatusdescribed in US. Patent No. 3,010,807, or other suitable apparatus. Whenthe gas stream passes through converter 14, contact with catalyst bed 16results in the water gas shift reaction be tween carbon monoxide andsteam, yielding carbon di- The resulting gas stream removed via 17 haslow or negligible carbon monoxide content, and is relatively high incontent of free hydrogen.

Stream 17 is now passed through catalytic hydrogenation unit 18, whichis provided with gauze layers 19 consisting of a platinum group metalsuch as platinum or palladium. As an alternative, unit 18 may beprovided with a catalyst bed consisting of platinum deposited on asuitable carrier. In any case, catalytic hydrogenation ofunsaturates inthe gas stream is carried out in unit 18. Thus in the case of ethylene,addition of hydrogen to the unsaturated double bond produces ethane.

The resulting process stream 20 withdrawn from unit 18 has a negligiblecontent of unsaturates. Stream .20 is now passed through scrubber 21,Where the gas stream is contacted with a liquid scrubbing agent such ashot aqueous potassium carbonate solution or aqueous monoethanolaminesolution, for removal of carbon dioxide. The scrubbing unit 21 isprovided with a packed section 22 for gas-liquid contact, alternativelybubble cap trays or grid trays may be provided for this purpose. Thescrubbing liquid solution is admitted via 23, and liquid solutioncontaining absorbed carbon dioxide is removed via 24 for externalregeneration.

The final gas stream of reduced carbon dioxide content is removed fromunit 21 via 25, and may be subsequently cooled or otherwise treated bymeans not shown for the removal of Water vapor. In any case, a finishedtown gas is produced, containing principally ethane, methane, hydrogenand nitrogen. .Nitrogen is a desirable component in finished town gas,since it serves as ballast in diluting the gas stream to provide theproper B.t.u. value. Due to thehydrogenation in unit 18, the finishedtown gas is low in unsaturates, which are undesirable since thesecompounds act as illuminants during combustion and also have gum-formingtendencies.

As an alternative embodiment of the present invention, a suitablecatalyst such as metallic nickel rings may be provided in unit 8, inorder to promote the formation of hydrogen by catalytic steam reform,thus yieldinga final gas product of higher B.t.u. value.

It has been found that operating pressure does not appear to be asignificant variable in the process of the present invention. Althoughpressure is notcritical, an operating pressure in the range of 1 to 22atmospheres is preferable since reform plant equipment size is reduced,and also become subsequent compression costs are reduced. In addition,gasification at elevated pressure yields a high pressure process gaswhich thus may range of 1450 F. to 1500 F. However, of a longerresidence interval up to 1.0 second is required, then the initialstreams 3, 4 and 5 must be preheated to higher levels so as to provide atemperature range of 1650 F. to 1690 F. in chamber 8, in order toprevent accumulated deposition of free carbon in actual operation of theprocess.

Similarly, it will be recognized that a minimum steam/carbon ratio of1.5 is generally required,in order to satisfy material balanceconsiderations by providing snflicient steam for complete reaction withthe naphtha. However, a steam/carbon ratio in the range of 3 to 6 hasbeen found to be optimum in providing complete reaction, satisfactoryreaction rate, and minimum tendency for carbon formation due to processupsets. Higher proportions of process air will generally berequired ifminimum steam/carbon ratios are adopted, in order to prevent carbondeposition. In general, the molar air/ carbon ratio Will range between0.1 and 1.0.

Following are examples of industrial application of the process of thepresent invention in the production of a stable combustible gas fromnaphtha.

TABLE I Production of crude combustible gas Run No 1 2 3 Steam/CarbonRatio 3. 6 4. 4 3. 6 Air/Carbon Ratio 0. 58 i 0.59 0.59 Reaction Time,sec.-- 0. 48 .0. 37 0. 30 Pressure, p.s.i.g 240 150 100 GasificationTemp Carbon Dioxide 9. 9 11.0 9. 6 Carbon M0noxid" 8. 5 7. 4 12. 5Ethylene. 5.8 7. 8, 7. 2 Methane 20. 5 16. 6 16. 0 Hydroge 27. 8 27. 631. 1 N itrogen 27. 5 29. 6 23. 6

Runs 1 and 2 were non-catalytic, while in run 3 a nickel type catalystwas provided in the gasification step. It is evident that ahighercontent of free liydrogen was present in the exit gas from run 3,thus the B.t.u. value was somewhat greater due to the catalytic effect.

Production of finished town gas be directly treated for carbon dioxideremoval by hot potassium carbonate scrubbing.

It will be evident to those skilled in the art that the a significantprocess variables in the gasification of naphtha according to thepresent invention are closely interrelated. T bus, the required minimumpreheat temperature of the reactant streams piror to chamber 8 willdepend principally on the residence time in 8 prior to entry of themixed stream via 9 into quench unit 10. With lower residence times inthe range of 0.05 to 0.10 second, it has been found that the process maybe successfully carried out with a residence chamber temperature in theThe final town gas product had a net heating value of 437 Btu/ft. and aspecific gravity of 0.502.

Following is an analysis of the commercial naphtha employed in the aboveruns. It is evident that naphthas of varying compositions andanalysesmay be successfully reformed by suitable selection of processvariables within the scope of the present invention.

TABLE HI Specification of tested naphtha Initial boiling point F.) Endpoint F.) 250 Specific gravity 60 F -Q. 0.70 Composition (wt. percent):

Parafiins 57.4 Naphthenes 32.5 Aromatics 9.4 Olefins 0.7 Sulfur content(p.p.rn.) -10 From the above analyses of the gas stream in Table I,certain conclusions may be reached with respect to probable reactionmechanism. Thus, since no oxygen is present, combustion of naphtha hasalready taken place to completion. Since some unsaturates as well asmethane are present, it is evident that some thermal cracking of naphthaalso took place, probably together with some hydrogenation of unstablefree radicals and unsaturated carbon linkages. This thermal crackingthus was accomplished without carbon accumulation. Finally some freehydrogen is also present hence non-catalytic (thermal) steam reform ofnaphtha also took place.

We claim:

1. A process for the gasification of naphtha to produce a combustiblegas which comprises vaporizing and preheating naphtha to a temperaturein the range of 122 F. to 1000" F., 'superheating steam, and preheatingair;

combining said streams of naphtha, steam and air .to

form a mixed gaseous stream at an initial temperature of at least 1000F., said mixed gaseous stream having a steam to carbon molar ratio of atleast 1.5 and an air to carbon molar ratio of at least 0.1, reactingsaid mixture for an interval less than 1.0 second whereby the processstream temperature rises to a minimum of at least 1400 F. and saidnaphtha is simultaneously oxidized, cracked and partially reformedwithout accumulated deposition of free carbon, and quenching the processgas stream to a temperature below 800 F. by contact with a liquid quenchagent, whereby a stable crude combustible gas mixture is producedcomprising carbon dioxide, carbon monoxide, ethylene, methane, hydrogenand nitrogen,

without accumulated deposition of free carbon.

2. The process of claim 1, in which said liquid quench agent is water.

3. A process forthe gasification of naphtha to produce a combustible gaswhich comprises vaporizing and preheating naphtha to a temperature of400 F. to 800 F., preheating air to a temperature above 800 F. andsupereating steam to a temperature above 1500 F.; combining said streamsof naphtha, steam and air to form a mixed gaseous stream at atemperature of 1400 F. to 1700 F. and a total pressure in the range or"1 to 22 atmospheres, said mixed gaseous stream having a steam to carbonmolar ratio in the range of 1.5 to 6.0 and an air .to carbon molar ratioin the range of 0.1 to 1.0,

reacting said mixture for a time interval between 0.05 to .0.33 second,whereby said naphtha is simultaneously oxidized, cracked and partiallyreformed Without accumulated deposition of free carbon, and quenchingthe process gas stream to a temperature below 800 F. by contact withliquid water, whereby a stable crude combustible gas mixture is producedcomprising carbon dioxide, carbon monoxide, ethylene, methane, hydrogen'ahd nitrogen,

without accumulated deposition of free carbon.

combining said streams of naphtha, steam and air to form a mixed gaseousstream at an initial temperature of at least 1000 F, said mixed gaseousstream having a steam to carbon molar ratio of at least 1.5 and an airto carbon molar ratio of at least 0.1, reacting said mixture for aninterval less than 1.0 second whereby the process stream temperaturerises to a minimum of at least 1400 F. and said naphtha issimultaneously oxidized, cracked and partially reformed Withoutaccumulated deposition of free carbon, quenching the process gas streamto a temperature below 800 F. by contact with liquid water, whereby acrude combustible gas mixture is produced comprising carbon dioxide,carbon monoxide, ethylene, methane, hydrogen and nitrogen, withoutaccumulated deposition of free carbon, catalytically reacting the carbonmonoxide content of the gas mixture with water vapor to form furtherhydrogen and carbon dioxide by contacting the gas mixture with a watergas shift catalyst comprising promoted iron oxide, catalyticallyhydrlogenating the ethylone content of the gas mixture to ethane, andscrubbing the gas mixture with a liquid absorbent to remove carbondioxide, whereby a stable combustible town gas is produced comprisingethane, methane, hydrogen and nirogen.

6. The process of claim 5, in which said reaction of vaporized naphthawith steam and air is carried out in the presence of a nickel catalyst.

'7. A process for the gasification of naphtha to produce a combustibletown gas which comprises vaporizing and preheating naphtha to atemperature of. 400 F. to 800 F., preheating air to a temperature of 800F. to 1200 F., and superheating steam to a temperature of 1500 F. to1700 F; combining said streams of naphtha, steam and air to form a mixedgaseous stream at a temperature of 1400 F. to 1700 F. and a totalpressure in the range of 1 to 22 atmospheres, said mixed gaseous streamhaving a steam to carbon molar ratio in the range of 3.0 to 6.0 and anair to carbon molar ratio in the range of 0.1 to 1.0, reacting saidmixture in the presence of a nickel catalyst for a time interval between0.05 to 0.33 second, whereby said naphtha is simultaneously oxidized,cracked and partially reformed without'accumulated deposition of freecarbon, quenching the process gas stream to a temperature below 800 F.by contact with liquid water, whereby a crude combustible gas mixture isproduced comprising carbon dioxide, carbon monoxide, ethylene, methane,hydrogen and nitrogen, without accumulated deposition of .tree carbon,catalytically reacting the carbon monoxide content of the gas mixturewith water vapor to form further hydrogen and carbon dioxide bycontacting the gas mixture with a Water gas shift catalyst comprisingpromoted iron oxide, catalytically hydrogenating the ethylene content ofthe gas mixture to ethane, and scrubbing the gas mixture with a liquidabsorbent to remove carbon dioxide, whereby a stable combustible towngas is produced comprising ethane, methane, hydrogen and nitrogen.

References Cited by the Examiner UNITED STATES PATENTS ldORRES O. W OLK,Primary Examiner,

1. A PROCESS FOR THE GASIFICATIN OF NAPHTHA TO PRODUCE A COMBUSTIBLE GASWHICH COMPRISES VAPORIZING AND PREHEATING NAPHTHA TO A TEMPERATURE INTHE RANGE OF 112*F. TO 1000*F., SUPERHEATING STEAM, AND PREHEATING AIR;COMBINING SAID STREAMS OF NAPHTHA, STEAM AND AIR TO FORM A MIXED GASEOUSSTREAM AT AN INITIAL TEMPERATURE OF AT LEAST 1000*F., SAID MIXED GASEOUSSTREAM HAVING A STEAM TO CARBON MOLAR RATIO OF AT LEAST 1.5 AND AN AIRTO CARBON MOLAR RATIO OF AT LEAST 0.1, REACTING SAID MIXTURE FOR ANINTERVAL LESS THAN 1.0 SECOND WHEREBY THE PROCESS STREAM TEMPERATURERISES TO A MINIMUM OF AT LEAST 1400* F. AND SAID NAPHTHA ISSIMULTANEOUSLY OXIDIZED, CRACKED AND PARTIALLY REFORMED WITHOUTACCUMULATED DEPOSITION